sb valve covers

having 2 breathers probably wont cause oil leaks. it's when you have a plugged pcv valve that it will spit oil out the single breather

but having a working pcv has a benefit... it pulls the vapors out one side, & a little fresh air goes into the breather down into the crankcase ... this sucks out a bit of the moisture

here is a very technical writeup if you are interested

http://www.burtonpower.com/tuning-guides/tuning-guide-pages/engine-breather-system.html